Microwave heating apparatuses of this general type are known. Thus, German Offenlegungsschrift No. 3 447 544 Al discloses an apparatus which comprises an elongate microwave treatment chamber having an endless conveyor belt passing therethrough. Microwave supply conduits project into the treatment chamber from above, each conduit communicating with a rspective microwave generator. A control device, which is connected to each microwave generator, switches the generators off whenever the edges of a foodstuff pack to be treated pass beneath the supply aperture, so that the risk of the edges of the pack becoming overheated is obviated or at least minimised. By means of, for example, a photocell or any other suitable device which is also connected to the control device, a signal is transmitted to the control device when the end faces of the foodstuff packs located on the conveyor belt, pass beneath the supply apertures of the microwave generators. However, such an arrangement cannot achieve uniform and rapid heating of the foodstuff packs.
German Patentschrift No. 1 291 429 discloses an apparatus for the dielectric heating of substances which apparatus includes a microwave generator and a high-frequency generator. Interchangeable field shapers are disposed at the ends of the waveguides facing the treatment chamber which permit the depth of penetration of the microwave energy into the product, that is to say, the concentration of the microwave energy, to be taken into consideration. However, a product cannot be uniformly heated over its entire area using such an apparatus, because controlling the depth of penetration presupposes that the concentration is at the desired level. Moreover, the depth of penetration is also dependent on the particular product being treated.